Septic tank systems are widely used in areas where municipal sewers are not available. In a typical installation, the waste water from a residence or commercial facility is directed into the septic tank, and here nonsoluble solids settle out of the flow (to be removed periodically when the tank is pumped out), while the suspended organic material undergoes anaerobic decomposition. Traditionally, the effluent from the septic tank then enters a drain field from which it is dispersed into the surrounding soil, with a certain amount of aerobic conversion taking place at this point. The waste water then migrates through the soil into the groundwater, or sometimes into adjacent surface water, such as a stream or lake.
Such traditional septic tank installations, although long utilized, exhibit several inherent limitations. Firstly, owing to the relatively inefficient manner in which aerobic treatment takes place in the drain field, this must be quite large, which is impractical in many instances; in fact, at many sites it is simply not possible to install a conventional drain field at all due to soil or topographical conditions. Also, large drain fields require significant excavation and a good deal of piping and other materials to install, and therefore involve high capital costs. Still further, conventional drain fields often become clogged and inoperative over a period of time, and if this happens, the capacity of the system may be greatly reduced.
One approach to overcoming these difficulties which has achieved considerable success involves the use of a secondary waste water treatment unit in which the effluent undergoes intensified aerobic processing before being discharged. An example of such a plant is the Model BP3 unit, available from Ekofinn Bioclere.TM., 3020 South 96th Street, Tacoma, Wash. The present invention constitutes an improvement to systems of this type, and so an overview of such secondary waste water treatment plants will be provided here with reference to FIG. 1. As can be seen, the overall system 10 comprises generally a septic tank 12 and a secondary treatment unit 14. Septic tank 12 is of a conventional configuration, this having a main tank 16 which receives waste water from the dwelling via an inlet line 18. As was noted above, the solids settle out of the waste stream in tank 16, and an access cover 20 is provided for periodic removal of the accumulated materials. As was also noted above, initial biological treatment of the waste water takes place in tank 16, in the form of anaerobic decomposition. The septic tank effluent is then discharged via effluent line 22, this being a "clear" fluid relatively free of suspended solids.
Effluent line 22 (which would lead directly to a drain field in a traditional arrangement) carries the effluent into the secondary treatment unit 14. This comprises generally a lower sump portion 24 and an upper aerobic treatment portion 26, both of these being enclosed in a generally cylindrical assembly. The effluent is discharged into a central, open-ended baffle 28, from which it is withdrawn by a submersible pump 30. The discharge from this passes upwardly through a pressure tube 32 and is discharged on top of a conical distribution plate 34. The effluent flows out over this to the perimeter of the plate, from which it falls in streams to suspended sprayer plates 36. These are positioned immediately above the surface of trickling filter media bed 40, and they break up the streams and distribute them over the top of the bed. In some versions, the conical distribution plate may simply be provided with a multiplicity of perforations to distribute streams of the liquid onto the bed.
The filter media bed is typically made up of a random packed material, such as, for example, a multiplicity of small disc-shaped plastic elements having radial and annular ribs to provide increased surface area. As is known to those skilled in the art, bacterial colonies adhere to the surfaces of these elements, so as to form a biological slime which decomposes the organic components of the effluent as this flows through the media bed. Oxygen is required for the aerobic reactions which are carried out by the bacteria in the bed, and this is provided by means of a fan 42 which blows air into a chamber 44 over the bed 40.
When the aerobically treated effluent has migrated to the bottom of the bed, it drips from ports in the lower pan 46 of the treatment chamber and returns to the sump 24, simply falling onto the surface of the liquid in the sump, outboard of the central baffle 28. When the liquid in the sump reaches the level of outlet port 47, this flows out through overflow line 48 and is discharged; the biological oxygen demand (BOD) of the effluent thus having been greatly reduced by the secondary treatment unit, this can be safely discharged into a small or reduced capacity drain field, or sometimes directly into ground or surface waters.
While secondary treatment units of this type have proven highly successful in many respects, they are not without their limitations. The first of these stems from the difficulty in providing an adequate supply of oxygen to promote vigorous growth of the bacterial slime on the trickle medium. Common agents for providing this biological slime include the bacteria Pseudomonas, and these may be seeded into the medium to facilitate bringing the system on line. However, as these bacteria grow so that the biological slime increases in thickness, the innermost layers (i.e., those closest to the underlying substrate) become starved of nutrients and especially oxygen; eventually, the innermost layers die, and the material sloughs off of the substrate and is flushed into the sump. Eventually, the biological slime reestablishes itself on the now bare substrate, but this takes a period of time.
Because the slime coat is thus weakened by the relatively poor oxygen supply, it is not able to resist significant shear forces. As a result, it has been found that such conventional secondary treatment units cannot be operated on a continuous basis, since the effluent flow tends to wash away the bacterial slime coat and so impedes the operation of the device. These systems are therefore limited to operating on an intermittent or periodic basis, so as to provide "rest periods" for the biological slime coat, and this reduces the system capacity below that which it would have if it could be operated on a continuous basis. Also, because the pump is subjected to relatively high start-up loading during the repeated on/off cycles, its energy usage is excessive, and it also must be somewhat larger and more expensive than would be needed if the pump could operate continuously. Also the cyclical loading subjects the pump to excessive wear.
Another limitation which is associated with such conventional secondary treatments units is that even if complete aerobic decomposition is successfully carried out, the effluent from the unit is still not ideally conditioned for discharge. The effluent from the primary treatment septic tank is rich is toxic ammonium (NH.sub.3); this is converted by the aerobic activity in bed 40, first to nitrites (--NO.sub.2), and then to nitrates (--NO.sub.3). While the nitrates are non-toxic, they present a pollution problem if discharged directly into the environment; for example, they act as a "fertilizer" to promote algae blooms and the like if they enter surface waters.
The nitrates are capable of being converted by biological activity to water and elemental nitrogen (N.sub.2), which can be safely discharged without environmental consequence, but this requires another anaerobic metabolic step. However, since the nitrate-rich effluent which drips from bed 40 into the sump 24 tends to cycle within a relatively oxygenated surface zone until it is ultimately discharged through the overflow line, little or no such anaerobic activity can take place, and so the discharge is likely to contain a high nitrate content.
Yet another limitation which is exhibited by such conventional secondary treatment units is that they do not address the problem of bacterial contamination of the effluent. The fecal colliform bacteria content of the effluent entering the secondary treatment unit from the septic tank tends to be very high, and in many instances, this presents a serious health hazard if it is discharged directly into the environment. Attempts have been made to deal with this by installing a separate sterilization facility downstream of the treatment unit, but this is obviously a complex and costly solution, and so is usually feasible only for relatively large installations.
Accordingly, there exists a need for a secondary treatment unit which provides for a more effective supply of oxygen to the biological slime on the trickle media so as to promote more vigorous growth and efficient aerobic action thereof. Furthermore, there exists a need for a such a unit which provides an effective anaerobic treatment phase for the material which has passed through the aerobic biological treatment provided by the trickle media bed. Still further, there is a need for such a unit which effectively sterilizes the effluent so as to minimize the fecal colliform content of the discharge from the unit.